Parameters

Mouse button actions are also handled by a dedicated input manager. Other programming features that rely on mouse button actions, such as input and command bindings, might invoke handlers for the action before it is exposed as a general mouse button action. If these input system features mark the Mouse::PreviewMouseDown event as handled, OnPreviewMouseDown is not invoked.

If you mark this event as handled in class handling, subevents are still raised; however, they pass the handled state in the event data. If the event is handled in class handling, instance handlers for the subevents are not invoked unless you explicitly use AddHandler(RoutedEvent, Delegate, Boolean) with handledEventsTootrue in order to attach handlers. Class handlers also are not invoked unless those class handlers were registered with the RegisterClassHandler(Type, RoutedEvent, Delegate, Boolean) signature with handledEventsTootrue. By handling OnPreviewMouseDown, you are implying that your class handling accounted for all possible mouse button down actions. This behavior might be unwanted; Therefore, use caution when you use this virtual method to mark events as handled.

Each of the button-specific direct events also has a virtual On* method; consider whether overriding these button-specific class handlers might be more appropriate.

This method has no default implementation. Because an intermediate class in the inheritance might implement this method, we recommend that you call the base implementation in your implementation.

The purpose of this method is similar to the common language runtime (CLR) event pattern On* methods: this method provides the means to handle the matching event from derived classes by establishing a class handler instead of an instance handler. In this case the matching event is a routed event. The implementation pattern of the On* methods is different for routed events because a routed event can be raised by a child element, which is not necessarily the element that will invoke handlers. Therefore, your implementation needs to examine the source properties of the event data. It should not try to reraise the event in most cases.

Either by overriding this method or by registering class handlers with RegisterClassHandler, derived classes of ContentElement can call private class handler methods when the event is received along the event route. One scenario where class handling is appropriate is to manipulate the event data and mark the routed event as handled. Another scenario that is specifically for Preview events is to mark them handled so that the matching bubbling class handlers are not invoked.